Optical distributed Bragg reflector (DBR) is a unipolar heterostructure which consists of many alternating layers of two semiconductors of the same conductivity type, e.g., n-type, p-type, or intrinsic, with different refractive index, each layer having a respective thickness of a quarter wavelength and refractive index different from the refractive index of adjacent layers. When the DBR is used for current conduction, as is the case in surface emitting lasers (SELs) or surface emitting diodes, constituent heterojunction band discontinuities impede the current flow, which is a highly undesired concomitant effect.
Surface emitting lasers (SELs) are attractive as being of a small area, being capable of low divergence output beams, being inherently single longitudinal mode, being capable of having threshold current comparable to edge emitting lasers and being producible by planar technology. DBRs consisting of stacks of periodic quarter wavelength layers forming pairs of layers (or periods) each consisting of a low and a high refractive index compound semiconductors are used in the SELs. While the index difference between adjacent layers provides high optical reflectivity, energy bandgap difference leads to interface discontinuities forming potential barriers in the heterointerfaces between the two adjacent constituent layers of the DBR structures. Since current transport in many surface emitting laser structures occurs across heterojunction barriers, these potential barriers impede the carrier flow in the DBR structures and result in series resistance which gives rise to thermal heating of the device and thus deteriorates the laser performance. Attempts were made to reduce the series resistance, which included linear grading, step grading, and superlattice grading the DBR structures. For example, see K. Tai et al., "Drastic reduction of series resistance in doped semiconductor distributed Bragg reflectors for surface-emitting lasers," Appl. Phys. Lett. 56 (25), Jun. 18, 1990, pages 2496-2498; R. S. Geels et al., "Low Threshold Planarized Vertical-Cavity Surface Emitting Lasers," IEEE Photonics Technology Letters, Vol. 2, No. 4, April 1990, pages 234-236; Federico Capasso et al., "Doping interface dipoles: Tunable heterojunction barrier heights and band-edge discontinuities by molecular beam epitaxy," Appl. Phys. Lett. 46 (7), Apr. 1, 1985, pages 664-666 and U.S. Pat. No. 4,794,440 issued on Dec. 27, 1988 to F. Capasso. However, none of these attempts lead to the elimination of heterojunction band discontinuities between two different materials of the same conductivity type in the DBR. Therefore, a process for optimization, that is elimination of the heterojunction band discontinuities in the DBR, is still desirable.